For expository convenience, the present invention is illustrated with reference to one particular application thereof, namely as a calibration tool for use with a spectrum analyzer. It should be recognized, however, that the invention is not so limited.
A spectrum analyzer typically employs a variety of signal processing stages between the signal input port and the signal analysis stages. These processing stages often include one or more wideband front end amplifiers, one or more mixers, and several stages of I.F. amplification and filtering. While these stages are advantageous in many respects, they unavoidably change, to a greater or lesser degree, the amplitude and phase composition of the original input signal due to their non-unity frequency responses. Most of this change is caused by the I.F. filters, which tend to roll off the amplitude of the signal near the I.F. passband edges and to shift the signal phase as a function of frequency over the bandwidth of the I.F. filter.
In a conventional spectrum analyzer, these phase and amplitude characteristics (referred to here as the instrument's frequency response) are of little concern because the instrument only analyzes signals in the center of its I.F. structure. This limited response is effected by bandwidth resolution filters that, in some instances, have bandwidths of only a few hertz, thereby permitting the instrument to resolve closely spaced signals. (To characterize the spectrum of a broad band signal with such a narrow filter, the instrument sweeps its local oscillator signal so that different spectral components of the input signal pass through the filter. The magnitude of the filtered output signal is presented on a display as a function of time and represents the signal's spectral distribution.) Since the narrow resolution filters are centered in the I.F. passband, the frequency response of the I.F. is of virtually no consequence.
(When the term I.F. is used herein, it should be understood to include all the frequency converting, filtering and amplifying stages between the analyzer input and the detector stages.)
More recently, advances in digital signal processing have permitted broadband signals to be sampled and analyzed coherently, rather than broken down and analyzed in time-sequential fashion in narrow spectral bands. In such instruments, the I.F. passband characteristics become important since they shape the signal spectrum and perturb the phase relationships between its components. Fortunately, it is relatively simple in digital instruments to further process the signal to compensate for these frequency responses. However, they must first be quantified.
In the prior art, this quantization process has taken a number of forms. Most common has been to determine the phase and amplitude response of the instrument input circuitry at one frequency: the center of the I.F. passband. A correction based on this measurement is then applied to all subsequent measurements. Other quantization techniques have sought to quantify the amplitude and phase responses at a variety of frequencies throughout the I.F. One such technique excites the instrument with a pseudo random noise sequence. Another uses a single stepped sinusoid.
The pseudo random noise technique is fast and permits measurement of phase data. However, it provides a generally poor signal-to-noise ratio, which compromises the accuracy of the measurement. The signal-to-noise ratio can be improved somewhat with averaging, but averaging requires additional time, negating the speed advantage.
The stepped sinusoid technique yields very good signal-to-noise ratio, but requires repeated measurements and does not readily provide phase data.
Both the pseudo random noise and stepped sinusoid excitation techniques require additional equipment not generally included with the signal analysis instrument.
It is a primary object of the present invention to provide a method and apparatus for quickly and accurately quantifying the frequency response of an instrument across its entire I.F. passband without use of additional equipment.
Briefly, this object is accomplished in the present invention by exciting the instrument with a fixed frequency signal from a calibration oscillator internal to the instrument. To yield data across the entire I.F. passband, the instrument's local oscillator frequency is swept, translating the fixed input signal to a signal that sweeps the I.F. passband. The instrument's own analysis stages examine the phase and amplitude characteristics of this swept I.F. signal and store the results in a calibration memory for later use.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.